A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (which will be referred to herein as “GaN-based semiconductors”) have been researched and developed particularly extensively. As a result, blue light-emitting diodes (LEDs), green LEDs, and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products (See, for example, Patent Documents 1 and 2).
A gallium nitride-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlaGabIncN (where 0≦a, b, c≦1 and a+b+c=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four primitive vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector c runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. It should be noted that the “c-axis” and the “c-plane” are sometimes referred to as “C-axis” and “C-plane”, respectively.
The wurtzite crystal structure has other typical crystallographic plane orientations than the c-plane, as shown in FIG. 3. FIG. 3(a) shows a (0001) plane. FIG. 3(b) shows a (10-10) plane. FIG. 3(c) shows a (11-20) plane. FIG. 3(d) shows a (10-12) plane. As used herein, “-” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). The (0001) plane, the (10-10) plane, the (11-20) plane, and the (10-12) plane are the c-plane, the m-plane, the a-plane, and the r-plane, respectively. The m-plane and the a-plane are “non-polar planes” that are parallel to the c-axis (primitive vector c), and the r-plane is a “semi-polar plane”.
For years, a light-emitting device in which a gallium nitride-based compound semiconductor is used is fabricated by means of “c-plane growth”. As used herein, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c, m, a, or r) of a hexagonal wurtzite structure. As for the X-plane growth, the plane will be sometimes referred to herein as a “growing plane”. Furthermore, a layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”.
When a light-emitting device is fabricated using a semiconductor multilayer structure formed by means of the c-plane growth, strong internal polarization occurs in a direction perpendicular to the c-plane (c-axis direction) because the c-plane is a polar plane. The reason for occurrence of the polarization is that, on the c-plane, there is a shift in the c-axis direction between the positions of a Ga atom and a N atom. If such polarization occurs in a light emitting section, a quantum confinement Stark effect of carriers occurs. This effect reduces the probability of radiative recombination of carriers in the light-emitting section and accordingly reduces the light emission efficiency.
In view of such circumstances, in recent years, intensive research has been carried out on growth of a gallium nitride-based compound semiconductor on a non-polar plane, such as m-plane and a-plane, and a semi-polar plane, such as r-plane. If a non-polar plane is available as the growing plane, no polarization occurs in the layer thickness direction (crystal growth direction) of the light-emitting section. Therefore, the quantum confinement Stark effect does not occur. Thus, a light-emitting device which potentially has high efficiency can be fabricated. Even when the growing plane is a semi-polar plane, the influence of the quantum confinement Stark effect can be greatly reduced.
Light-emitting diode products commercially available in the present market are manufactured by mounting to a submount a light-emitting diode element (LED chip) which is fabricated by epitaxially growing a GaN-based semiconductor layer, such as GaN, InGaN, AlGaN, or the like, on a c-plane substrate. The planar size of a light-emitting diode element (the planar size of the principal surface of the substrate: hereinafter, simply referred to as “chip size”) varies depending on the use of the light-emitting diode element. Typical chip size is, for example, 300 μm×300 μm or 1 mm×1 mm.
The arrangement of the electrodes of the light-emitting diode element can be generally classified into two types. One is the “opposite-surface electrode type” wherein the p-electrode and the n-electrode are provided on the front surface and the rear surface, respectively, of the light-emitting diode element. The other one is the “front-surface electrode type” wherein both the p-electrode and the n-electrode are provided on the front surface of the light-emitting diode element. Hereinafter, the configurations of prior art light-emitting diode elements which have such electrode arrangements will be described.
FIG. 4(a) is a cross-sectional view showing a light-emitting diode element of the opposite-surface electrode type. FIG. 4(b) is a perspective view of the light-emitting diode element of the opposite-surface electrode type. FIG. 4(c) is a cross-sectional view showing the light-emitting diode element of the opposite-surface electrode type which is mounted on a mounting base 12. FIG. 5(a) is a cross-sectional view showing the light-emitting diode element of the front-surface electrode type which is mounted on the mounting base 12. FIG. 5(b) is a side view of the light-emitting diode element of the front-surface electrode type, which is seen from the side including a p-electrode 5 and an n-type front surface electrode 6.
In the example shown in FIG. 4(a) and FIG. 4(b), a multilayer structure is provided on an n-type substrate 1 which is made of GaN. The multilayer structure includes an n-type conductive layer 2 which is made of GaN, an active layer 3 which is made of a quantum well of InGaN and GaN, and a p-type conductive layer 4 which is made of GaN. The p-electrode 5 is provided on the p-type conductive layer 4, and an n-type rear surface electrode 7 is provided on the rear surface of the n-type substrate 1. In this example, light emitted by the active layer 3 is extracted through the rear surface of the n-type substrate 1. As such, the n-type rear surface electrode 7 is made of a transparent electrode material. When the n-type rear surface electrode 7 is made of a nontransparent conductive material, the n-type rear surface electrode 7 is provided in part of the rear surface of the n-type substrate 1 so as not to block the light. When mounting a light-emitting diode element of the opposite-surface electrode type wherein the n-type rear surface electrode 7 is transparent, the p-electrode 5 is arranged so as to be located on the mounting base 12 side as shown in FIG. 4(c). On the n-type rear surface electrode 7, a bonding pad 15 is provided. The bonding pad 15 is electrically coupled to the mounting base 12 via a wire 16.
In the example shown in FIG. 5(a) and FIG. 5(b), the p-type conductive layer 4, the active layer 3, and the n-type conductive layer 2 are partially removed, and the n-type front surface electrode 6 is provided on the exposed part of the n-type conductive layer 2. The p-electrode 5 is provided on the p-type conductive layer 4. In this example, light generated in the active layer 3 is extracted through the rear surface of the substrate 1. Therefore, when mounting a light-emitting diode element of this type, the diode element is mounted such that the p-electrode 5 and the n-type front surface electrode 6 are located on the mounting base 12 side.
In the case of the opposite-surface electrode type, the electric resistance between the p-electrode 5 and the n-type rear surface electrode 7 is greatly affected by the resistance component of the GaN substrate 1. Therefore, it is preferred to reduce the resistance of the GaN substrate 1 as small as possible. The GaN semiconductor is doped with an n-type impurity at a relatively high concentration than a p-type impurity. Therefore, in general, a low resistance is realized more readily with the n-type impurity. Thus, commonly, the conductivity type of the GaN substrate 1 is set to the n-type.
Also, in the case of the front-surface electrode type, the electric resistance between the p-electrode 5 and the n-type front surface electrode 6 is affected by the resistance component of the GaN substrate 1. Therefore, commonly, the conductivity type of the GaN substrate 1 is set to the n-type.
The above-described electrode arrangements have been employed in c-plane light-emitting diode elements, and they are also applicable to m-plane light-emitting diode elements without modification.